The present invention relates to a process for making asphalt concrete from aggregate, such as stone and sand, and binder material, such as asphalt cement. Other additives may be included.
Current and prior processes and apparatus for making asphalt concrete include direct-fired processes and apparatus and indirect-fired processes and apparatus. Direct-fired processes generally are of two types. In one, aggregate is directly heated, as by a flame, and the heated aggregate is mixed with a binder to form the asphalt concrete. This is a batch process. In a second process, a continuous process, a mixture of aggregate and binder is directly heated, usually by an open flame burner. In indirect-fired processes, the mixture within a mixing apparatus is indirectly heated by means of a heat transfer fluid.
The following U.S. patents disclose processes and/or apparatus using the direct-fired technique: U.S. Pat. No. RE. 29,496 of Dydzyk, U.S. Pat. Nos. 1,984,315 of Morris, 2,256,281 of Finley, 2,487,887 of McEachran, and 3,840,215 of McConnaughay. With prior art systems and particularly direct-fired systems, significant amounts of hydrocarbons, such as polycyclic organic materials which include suspected carcinogens, particulate matter and the like are exhausted from the apparatus and vented into the atmosphere.
There have been some attempts to reduce the particulate pollutants, for example, the system set forth in U.S. Pat. No. RE. 29,496. This patent discloses that the exhaust gases from the direct-fired mixer are recycled through the mixer after first passing through a heat exchanger and dust separator. U.S. Pat. No. 3,840,215 discloses passing exhaust gases containing dust particles and other particulate solids into knock out boxes where the dust and solid particles are removed before the gases are exhausted. However, the production and emission of non-particulate pollutants are not controlled by these devices and processes.
Moreover, no attempt is generally made to maintain moisture in asphalt concrete and to control the amount of moisture in asphalt concrete within the predetermined limits as set forth hereinafter. The high heat associated with the direct-fired mixers drives substantially all of the free and combined water from the product, in contrast to the present invention wherein some moisture remains in the asphalt concrete product.
Some moisture can be retained within the product made in prior art direct and indirect-fired mixing apparatus by reducing the final mixture temperature. Any retained moisture is purely a function of temperature, since pressure cannot be controlled in prior art processes and apparatus. The present invention overcomes problems relating to control of moisture content at any and all temperatures by controlling both temperature and pressure.
Two general types of indirect-fired apparatus used for heating and mixing asphalt concrete are known. In one type, the entire mixing chamber is rotated, similar to the direct-fired apparatus, but the heat is provided by indirect heat-exchange fluid contained in tubes or pipes distributed throughout the rotating mixing drum. Typical processes and apparatus wherein heat exchange occurs in tubes within the rotating drum of the mixing chamber include those disclosed in the following U.S. Pat. Nos. 2,715,517 of Bojner and 3,845,941, 4,000,000, 4,067,552 and 4,074,894, all of Mendenhall. Mendenhall's patent 4,074,894 discloses an indirect-fired mixer wherein water vapor and hydrocarbon gases evaporated from the heated mixture are withdrawn from the mixing chamber in a stream of air. The water vapor withdrawn with the hydrocarbons and air is condensed and removed from the mixture. The remaining gases from the heated mixture are recycled, along with air, to the combustion chamber for combustion and eventual discharge to the atmosphere. Thus, while some attempt is made in this patent to reduce pollutants, it is believed that a significant quantity remain due to the exhaustion of the combustion of gases formed by the mixture into the atmosphere. There has been no attempt to control the moisture content of the product when using these indirect-fired mixers. It should be noted that effective control of moisture in the product is not possible at atmospheric pressure.
Another type of indirect-fired apparatus that could be used for making asphalt concrete comprises a mixing chamber wherein the mixture is mixed and heated by screw conveyors having hollow flights and at least one hollow shaft containing a heat exchange material. Several different embodiments of this type of apparatus are described in the following U.S. Pat. Nos. 1,717,465 of O'Meara, 2,721,806 of Oberg et al., 2,731,241 of Christian, 3,020,025 of O'Mara, 3,056,588 of Alexandrovsky, 3,250,321 of Root 3rd, 3,263,748 of Jemal et al., 3,285,330 of Root 3rd, 3,486,740 of Christian, 3,500,901 of Root 3rd et al., 3,765,481 of Root, and 4,040,786 of Christian. The only patent of this group which discloses a process or apparatus for making asphalt concrete is 2,731,241.
The patents relating to the indirect-fired apparatus using hollow flights, hollow shaft screw conveyors to mix and heat the mixture generally suffer from the same inherent disadvantages of the other type of indirect-fired apparatus. These disadvantages include venting of gases produced by heating the mixture to the atmosphere and failure to adequately control the moisture content of the mixture.
The prior art systems, both the direct and indirect-fired systems, generally operate at high temperatures to produce an asphalt concrete product having a discharge temperature of about 121.degree.-154.degree. C. (250.degree.-310.degree. F.) and require large amounts of energy. None of the prior art systems has recognized the energy value of moisture contained in the aggregate and/or binder used to make asphalt concrete. Instead of using the energy in the entrained moisture, the prior art systems use more energy to drive off the moisture, typically about 20-50% of the energy used. There is no recognition that any particular amount of moisture in the final product results in a superior product, contrary to the present invention.
The present invention is based upon the discovery that the strength and specific gravity or density of hot mixed asphalt concrete can be increased by controlling the moisture content of the asphalt concrete during mixing within prescribed limits defined by the environmental conditions and the moisture content and absorption of the starting materials. Strength and density both affect the useful life and durability of asphalt concrete when used for its normal purposes, for example in highways, driveways, parking lots and the like.